Piezoelectric motors that are able to move a load from a first position to a second position relatively rapidly, even when moving the element requires translating and/or rotating the load by a relatively large distance and/or angle, are known in the art. Generally, such a piezoelectric motor comprises a piezoelectric vibrator having electrodes on surfaces thereon and a power supply that electrifies the electrodes with an AC voltage to generate high frequency elliptical vibrations in the vibrator. The vibrator has a coupling surface, often a “friction nub” formed from a relatively hard, wear resistant material mounted to a surface of the vibrator, which is resiliently pressed to a surface of a load that the motor moves.
To move the load, the power supply electrifies the electrodes to excite vibrations in the vibrator and as a result vibrations in the coupling surface. During each vibration cycle of the vibrator, vibratory motion of the coupling surface steps the load by a relatively small distance in a desired direction towards the second position. As a result of the high frequency of the vibrations, the motor moves the load by a large number of steps each second and thereby at a relatively high velocity.
However, there is often a need for such motors to be able to position a load that they move to an accuracy that is better than a small number of nanometers and controlling such motors to provide the required positioning accuracy is generally difficult. In addition, once a piezoelectric motor has positioned a load it is often required to reposition the load by moving it by accurately controlled nanometer distances equal to or less than a few nanometers or tens of nanometers or sub-nanometer distances. Controlling a piezoelectric motor to provide such performance is also generally difficult.
U.S. Pat. No. 5,616,980, the disclosure of which is incorporated herein by reference, describes electrifying electrodes of a piezoelectric motor with unipolar pulses to control the motor to accurately position a load at a desired location. U.S. application Ser. No. 09/980,375, the disclosure of which is incorporated herein by reference, describes controlling the step size of a piezoelectric motor by controlling motion of the motor's coupling surface perpendicular to the load independently of motion of the coupling surface parallel to the load to accurately position the load.